Development of a functional and efficient nervous system requires the orchestrated migration and differentiation of axons and their associated glia. Motor axons connect the central nervous system (CNS) with targets in the periphery, including muscle. These axons interact with myelinating glial cells both in the CNS and peripheral nervous system (PNS). Ultimately, the differentiation of these two distinct glial populations forms a specialized structure known as the transition zone (TZ), which exists at every boundary between the spinal cord and periphery. Interestingly, at motor exit point (MEP) TZs, oligodendrocytes and peripheral myelinating glia normally stay restricted to their respective half of the nervous system, while other glia, including perineurial glia and a newly described population of cells, MEP glia, freely migrate from the spinal cord out into the periphery. How MEP TZs are selectively permeable, restricting myelinating cells from mixing, while allowing the passage of other populations, is unknown. In this project, we will characterize the development and function of a novel population of glia, motor exit point (MEP) glia, that we demonstrate are essential for restricting oligodendrocyte progenitor cells (OPC) to the spinal cord (Aim 1). In Aim 2, using both a candidate and unbiased approach, we will investigate the molecular mechanism that mediates MEP glia-OPC interactions during development. Defects in the development or maintenance of myelin along axons are the cause of many disorders collectively known as myelinopathies, one such example being Charcot-Marie-Tooth Disease (CMT). Some of the most severe types of this disease lead to demyelination, neurodegeneration and subsequent muscle atrophy in young children. Utilizing an in vivo system, zebrafish, to directly investigate the glial-glial interactions that establish MEP TZs, we will provide important insights into how functional nervous systems are assembled, maintained and behave during disease.